In many communication applications, it is often desired, or required, to spread the frequency spectrum of a transmitted signal by a given factor. For example, in the United States, the Industrial, Scientific and Medical (ISM) frequency band has historically been reserved for the operation of industrial, scientific and medical instruments. Recently, however, the Federal Communications Commission (FCC) has indicated that the ISM band may be utilized under certain conditions for communication applications, such as local area networks (LANs). Specifically, in order not to interfere with the operation of industrial, scientific and medical devices, the FCC requires that the frequency spectrum for communications in the ISM band be spread by a factor of at least 10. This is typically accomplished by encoding each bit to be transmitted using a predefined codeword, or pattern, consisting of at least 10 "chips" or "signal elements" which are all transmitted in the time frame normally allocated for a single bit.
The Institute for Electrical and Electronic Engineers has developed a standard for communications in the ISM band that utilizes the well known Barker code having a defined pattern of eleven chips, namely, "00011101101", as the basic information carrier. Thus, the Barker code may be utilized to represent a value of binary "0" and the inverse of the Barker code may be utilized to represent a value of binary "1", or vice-versa. Accordingly, for each transmitted eleven chip Barker code, one bit of information is conveyed.
Frequently, such spread spectrum systems will increase the bit rate by transmitting a number of bits during a single bit duration by utilizing phase-shift keying (PSK) modulation, wherein the phase of the carrier signal is shifted to represent data. In a quadrature phase-shift keying (QPSK) implementation, phase shifts in multiples of 90.degree. are utilized. Thus, four possible signal states may be represented by each of the four phases. Accordingly, each signal element can represent two bits rather than one.
Although additional gains in the bit rate could be achieved by extending these phase shifting schemes, for example, to transmit three bits per signal element, by providing eight phase angles, the increase in transmission power that would be required to achieve adequate error rate performance presently makes such schemes impractical.
As is apparent from the above discussion of conventional spread spectrum modulation techniques, a need exists for a spread spectrum modulation technique that increases the number of bits that are transmitted per signal element without significantly adding to the power requirements of the transmitter.